1. Field of the Invention
This invention relates to an LD/PD (laser diode/photodiode) module or an LED/PD (light emitting diode/photodiode) module as a sending/receiving apparatus used at base ports (broadcasting station) and subscribers in a unidirectional or bidirectional optical communication system which transmits optical signals of different wavelength in a unidirectional direction or bidirectional directions. In particular, this invention relates to an LD/PD or an LED/PD module which ensures facile attachment to and detachment from an optical connector. The word "LD/PD module" is adopted to signify both an "LD/PD module" and an "LED/PD module" hereafter for simplicity. At a subscriber port, an incoming light is called "receiving signal light" and an outgoing light is called "sending signal light".
This application claims the priority of Japanese Patent Application No.10-58806(58806/1998) filed Feb. 23, 1998 which is incorporated herein by reference.
2. Description of Related Art
[EXPLANATION OF BIDIRECTIONAL OPTICAL COMMUNICATION]
Recent development of technology has reduced transmission loss of optical fibers and has improved the properties of semiconductor laser diodes (hereafter indicated as LDs) and semiconductor photodiodes (hereafter indicated as PDs). The improvements of fibers, LDs and PDs enable us to transmit various types of information by light signals. The transmission is called "optical communication", since light signals carry information. The types of information for sending or receiving at stations or at subscriber ports are, for example, telephones, facsimiles or televisions. In particular, people have vigorously tried optical communication based on long wavelength light (near infrared), for example, of a 1.3 .mu.m wavelength or of a 1.55 .mu.m wavelength. Recently bidirectional transmission attracts attention, since only a single optical fiber can send signals both in a forward direction and in a backward direction at the same time. The system of the communication is called a "bidirectional communication system". Fortunately the bidirectional system saves one optical fiber.
FIG. 1 schematically indicates a multiwavelength bidirectional optical communication system which adopts a plurality of wavelengths for sending signals simultaneously both in a forward direction and in a backward direction.
One station is connected to a plurality of subscribers (ONUs) by optical fibers. Although FIG. 1 shows only a single subscriber for drawing convenience, many subscriber ports are connected to the central station. The fiber from the station branches at many bisecting points into a plurality of fibers linking with individual subscribers.
The central station amplifies the signals of telephones or televisions as digital signals or analog signals and drives a semiconductor laser (LD1) which produces .lambda.1 light responsive to the amplified signals. The light of .lambda.1 emitted from the LDI (P1) enters an optical fiber 1 as light signals of .lambda.1. A wavelength division multiplexer (WDM) 2 introduces the .lambda.1 light into an intermediate optical fiber 3. Another wavelength multiplexer (WDM) 4 allocates the .lambda.1 light to an optical fiber 5. A photodiode (PD2) senses the .lambda.1 signals for converting the optical signals to electric signals (P3). A receiver apparatus on the subscriber side amplifies and processes the electric signals (P3) for reproducing a voice or image. The signals flowing from the station to the subscribers are called "downward signals". The direction is called a "downward direction".
On the contrary, a subscriber converts electric signals of a facsimile or a telephone into .lambda.2 light signals by a semiconductor laser diode (LD2) which oscillates at a wavelength .lambda.2 (P4). Going into a fiber 6, the .lambda.2 light passes the WDM 4, propagates in the intermediate optical fiber 3 to the station. The WDM 2 allocates the .lambda.2 light into a fiber 7. A photodiode (PD1) senses the .lambda.2 light for converting into electric signals (P2). Converters or signal processing circuits on the station side regenerate telephone voice or facsimile images. The direction of the signal flow from the subscribers to the station is called an "upward direction". The signals are called "upward signals".
The above system appropriates .lambda.1 to downward signals and .lambda.2 to upward signals exclusively. Another system uses only one wavelength in common for both upward and downward signals. A further system doubly allocates two wavelengths .lambda.1 and .lambda.2 both for upward and downward signals. Separation of two wavelengths is an important problem in the optical communication system which carries different wavelength signals in an optical fiber.
[Explanation of wavelength division multiplexer]
Both the station and the subscribers require a device for discriminating wavelengths and separating one wavelength from others. A WDM is a device having such a function. In FIG. 1, the WDMs 2 and 4 play the role of distinguishing and separating different wavelengths. A WDM either joins .lambda.1 to .lambda.2 for introducing them into a fiber or extracts only one wavelength light from two wavelengths propagating in a fiber. WDMs play an important role in multiwavelength bidirectional optical communication systems.
Various types of wavelength division multiplexers have been suggested. FIG. 2 indicates a WDM constructed by optical fibers or optical waveguides. Two optical paths 8 and 9 lie in a close relation at a part 10 for allowing the exchange of power. The distance D and the length L of the close portion 10 determine the modes of coupling. In the example, when .lambda.1 enters the path 8 (P1), the same wave appears in a path 11 (P3). Going into a path 12 (P4), .lambda.2 appears in a path 9 instead of the path 8 (P2). The coupling portion 10 gives wavelength selectivity to the device.
FIG. 3 shows another WDM which uses a multilayered mirror. The WDM consists of two rectangular isosceles triangle columns 13 and 14, and a dielectric multilayer mirror 15 formed on the slanting plate of the columns. The two columns are glued at the dielectric mirror 15 for making a square column. Selection of the refractive index and the thickness gives the dielectric multilayer the wavelength selectivity of allowing one wavelength .lambda.1 shooting at 45 degrees to the multilayer to pass through and of reflecting another wavelength .lambda.2 at a right angle. This dielectric layer type WDM can be the WDMs 2 and 4 in the optical communication system of FIG. 1. The WDM is sometimes called a wave-division-integration device. Fiber-type WDMs and glass block type WDMs are already on sale.
An example of an LD/PD module on a subscriber side is explained by referring to FIG. 16. A single mode optical fiber 16 spreading from the central station is connected by an optical connector 17 to an optical fiber 18 of a subscriber (ONU) module. The ONU module has a fiber-type WDM 21 which couples the fiber 18 to a fiber 19 with wavelength selectivity. A contact portion 20 exchanges light power. An optical connector 22 couples the fiber 18 to an LD module 25 in the ONU. Another optical connector 23 joins the fiber 19 to a PD module 27.
The LD module 25 and the fiber 24 are part of an upward system. 1.3 .mu.m light carries signals from the subscriber to the station through the upward system. The fiber 26 and the PD module 27 are part of a downward system. The station sends 1.55 .mu.m light carrying signals to subscribers through the downward system. The PD module 27 converts the optical signals into electric signals. The LD module 25, a signal sending device, includes an electric circuit for amplifying and modulating the signals of telephones and facsimiles, and a laser diode (LD) for converting electric signals into optical ones. The PD module 27, a receiving device, contains a photodiode for converting optical signals from the station into electric signals, an amplification circuit for amplifying the optical signals and a demodulation circuit for restoring the television signals or telephone signals. The WDM 21 has a function for separating 1.55 .mu.m light from 1.3 .mu.m light. This example allots 1.3 .mu.m light to the upward system and 1.55 .mu.m light to the downward system.
This invention provides an improved LD/PD module for bidirectional multiwavelength optical communication. The LD/PD module signifies a device having an LD for producing optical signals, a PD for receiving optical signals and peripheral electric circuits for amplifying, modulating or demodulating signals. Related art is explained with regard to the parts.
[EXPLANATION OF KNOWN SEMICONDUCTOR LASER DIODE]
FIG. 4 shows a known laser diode device 28. The device 28 includes a semiconductor laser diode (LD) chip 29 and a monitoring photodiode (PD) chip 30. The laser diode chip 29 is vertically mounted on a side surface of a pole 31 on a header 32. The laser diode 29 emits light in a vertical direction. Photodiode chip 30 is mounted upon the top of the header 32 at a spot at which the laser shoots a backward light. A plurality of lead pins 33 implanted on the bottom of the header 32. A cap 34 covers the top surface of the header 32.
The cap 34 has a window 35 at the center. The light beams are emitted in both directions vertically from the laser diode 29. A lens 37 is fixed just above the window 35 by a lens holder 36. A conical housing 38 covers the top of the lens holder 36. A ferrule 39 pierces through a hole of the housing 38. The ferrule 39 holds an end of an optical fiber 40. The ends of the ferrule 39 and the fiber 40 are polished in a slanting plane of about eight degrees for impeding reflected light from going back into the laser. The holder 36 is aligned at an optimum spot to the header 32 by sliding the holder 36 and measuring the light power at the other end of the fiber 40. The housing 38 is also aligned to the lens holder 36 by the same manner. Wires connect the pads of the laser diode chip 29 and the photodiode chip 30 to the lead pins 33.
The lens 37 converges the light beams emitted from the laser 29 on the end of the fiber 40. The light beams go into the fiber 40. Since the laser is modulated by a driver circuit with an electric signal, the light carries the signal. The output of the laser diode is monitored by the monitoring photodiode 30. The material of the laser determines a wavelength between 1.3 .mu.m and 1.55 .mu.m of the light produced by the laser.
[EXPLANATION OF KNOWN PHOTODIODE DEVICE]
FIG. 5 shows a known photodiode device. The photodiode device has a header 42 as a package base. A photodiode chip 41 is die-bonded upon the header 42. Lead pins 43 project from the bottom of the header 42. A cap 44 protects the top surface of the header 42. An opening 45 is perforated at the center of the cap 44. A cylindrical lens holder 46 encircles the cap 44 upon the header 42. The lens holder 46 holds a lens 47 just above the photodiode chip 41.
The lens holder 46 has a conical housing 48 on the top. A ferrule 49 grips an end of an optical fiber 50. The ferrule 49 pierces a top hole of the housing 48 for fixing the fiber end to the housing 48. The ends of the ferrule 49 and the fiber 50 are slantingly polished for suppressing reflected light from returning back to the laser diode.
The holder 46 and the housing 48 are aligned at optimum spots by introducing light into the fiber from the farther end, measuring the light power by the PD 41 and maximizing the power input in the PD 41. The material of the light receiving layer of the PD 41 determines the range of wavelengths of the light detectable to the PD device. Silicon photodiodes are available for sensing visible light. However, silicon photodiodes are irrelevant for the present invention which aims at building an LD/PD module relying upon infrared light. Sensing of infrared light requires compound semiconductor photodiodes having InP as a substrate which has a relevant band gap and an InGaAs or an InGaAsP light receiving layer which has a narrow band gap for absorbing near infrared light.
Prior proposed LD/PD modules are not yet. Subscribers would be mostly households. Thus, the optical communication network should have a market as wide as telephones. However, consumers would not buy such an LD/PD module for optical communication unless the price of the module fell to a level as low as a telephone. For optical communication to become prevalent the subscriber sending-/receiving device must be inexpensive. The prior LD/PD apparatus proposed in FIG. 16 cannot be made as inexpensive as the price of a telephone, since the apparatus is only an assembly of a PD module, an LD module and a WDM module. The price is the sum of the modules.
A high price impedes the sending-/receiving devices from becoming prevalent among subscribers. Efforts have been to made to reduce the number of parts or diminishing the size of such devices for lowering their cost. Attempts for lowering the cost are now explained.
[A. Spatial separating beam type module (a receptacle containing a WDM, a PD and an LD)]
FIG. 6 shows a candidate proposed by Masahiro Ogusu, Tazuko Tomioka and Sigeru Ohshima, "Receptacle Type Bi-directional WDM Module I", Electronics Society Conference of Japanese Electronics, Information and Communication, C-208, p208(1996). The module is built in a rectangular housing 60. A WDM filter 61 is centrally positioned in a 45 degree slanting direction to sides of the housing 60. Three drum lenses 62, 63 and 64 lie in radial directions at three sides of the housing 60. A photodiode 66 is mounted on one side in front of the drum lens 62. A laser diode 68 is fixed on another side in front of the lens 63. The lens 64 lies at the end of the fiber 69.
In practice, the module consists of two separable parts. One part having the fiber end can be detached from and attached to the other part (housing 60) containing the filter 61, the PD 66 and the LD 68. In the coupled state, an external fiber 69 is connected via the WDM 61 to the PD 66 and the LD 68. The lenses 64, 62 and 63 prevent the light emanating from the fiber 69 from diverging spatially in the receptacle. The LD 68 emits 1.3 .mu.m light which penetrates the WDM filter 61 slantingly and goes into the fiber 69 for sending optical signals.
The incoming light propagating in the fiber 69 is 1.55 .mu.m light which is reflected by the WDM filter 61. The reflected light is transmitted via the lens 62 into the PD 66. The module of FIG. 6 is smaller than the previously explained module of FIG. 16. However, the receptacle type module still contains two independent optoelectronic devices of the LD and the PD and the indispensable WDM filter in addition to three lenses for preventing light from dispersing spatially. The alignment of parts is still as difficult as the module of FIG. 16. The cost is similar to the cost of FIG. 16.
[B. Y-bisecting wave guide type module (FIG. 7)]
FIG. 7 shows another LD/PD module proposed by Naoto Uchida, Yasufumi Yamada, Yoshinori Hibino, Yasuhiro Suzuki & Noboru Ishihara, "Low-Cost Hybrid WDM Module Consisting of a Spot-Size Converter Integrated Laser Diode and a Waveguide Photodiode on a PLC Platform for Access Network Systems", IEICE TRANS. ELECTRON., VOL.E80-C, NO.1, P88, January 1997. An insulating silicon substrate 70 is adopted as a base. A transparent quartz optical waveguide 71 is produced on the insulating silicon substrate 70. A corner of the waveguide 71 is cut into a step 72. Doping of impurity makes narrow Y-bisecting paths 73, 74, 76, 77 and 78 on the waveguide 71.
This example has two Y-branches. The first Y-branch has a WDM 75 at the cross point. The WDM 75 has wavelength selectivity that reflects 1.55 .mu.m light but allows 1.3 .mu.m light to pass through. Electrode patterns 79, 80, 81 and 82 are evaporated upon the step 72 of the waveguide. A light source 83 which has a bottom electrode is bonded on the electrode patterns 79 and 80. The light source 83 is either an LED or an LD for emitting 1.3 .mu.m light from a point 85 on an end surface.
A PD chip 84 is bonded on the farther electrodes 81 and 82 on the step 72 for sensing 1.3 .mu.m light. The light spreading in the optical fiber 88 contains both 1.3 .mu.m light and 1.55 .mu.m light. The light goes into the path 74 and attains to the WDM 75 which reflects 1.55 .mu.m light to another fiber 87. 1.3 .mu.m light continues along its way to the Y-branched paths 77 and 78. The half light reaching the LED 83 is of no use. The other half power shoots the PD 84 at a side point 86 for detecting the sent signals. The LED (or LD) makes sending signal light of 1.3 .mu.m which propagates in the path 78, the WDM 75 and the path 74 and enters the fiber 88.
This example uses the WDM only for excluding 1.55 .mu.m light. The biggest drawback of the proposed module is the difficulty of producing the planar Y-branched waveguide. Fabrication of curved Y-branch on a waveguide layer is far more difficult than the production of a straight path on a waveguide. In addition, the junction of fibers to the ends of the paths 73 and 74 is also a difficult task. Thus, the proposal gives us no means for solving the problem of making inexpensive LD/PD modules yet.
[C. Upward reflection WDM type LD/PD module (FIG. 8)]
FIG. 8 shows another LD/PD module proposed by Tomoaki Uno, Tohru Nishikawa, Masahiro Mitsuda, Genji Tohmon, Yasushi Matsui, "Hybridly integrated LD/PD module with passive-alignment technology", 1997 Conference of Electron, Information, Communication Society, C-3-89p198(1997). Mounting an LD and a PD on a common substrate allows the module to reduce cost and minimize size. In FIG. 8, a silicon substrate 90 provides a base to the module. Straight scribing makes a longitudinal V-groove 91 in the middle on the Si substrate 90. An end of an optical fiber 92 is inserted and fixed in the V-groove 91. Slanting cutting makes a deep slanting notch 93 midway across the fiber 92 and the V-groove 91 into the Si substrate 90. A fiber end 94 is cut and separated from the fiber 92 by the notch 93. The slanting notch 93 holds a WDM filter 95 in it. A PD 96 is mounted before the slanting WDM filter 95 above the V-groove 91 on the top of the Si substrate 90. A step part 97 is made by cutting the back end of the Si substrate 90. An LD chip 98 lies upon the step part 97. The LD 98 emits 1.3 .mu.m light 99 for sending signals. The sending light 99 propagates in the fiber 92 and the WDM 95 to the station (not shown). The receiving light of 1.55 .mu.m 100 running in the fiber 92 is reflected by the WDM 95 to be light 101 and sensed by the PD 96. The light path turns upward at the branch.
The module of FIG. 8 seems to have a simple structure. However, it is difficult to bury a fiber into a V-groove, and to align an LD and a PD to the fiber by sliding the LD and the PD and monitoring the light power sensed by the PD. A single-mode fiber has a 10 .mu.m core diameter and a 125 .mu.m cladding diameter. Insertion of the WDM filter 95 requires the notch to cut the thick cladding. Thus, two fibers are equivalently separated by (125 .mu.m+WDM) at the notch 93. The wide separation increases the reflection loss between the fibers 94 and 92. The LD light leaks at the gap.